1. Field of the Invention
The present invention relates to a device to deliver or administer medication to a patient, and more particularly, concerns a syringe pump monitoring circuit intended for the controlled delivery or administration of such medications.
2. Background Description
Hospitals typically use intravenous (I.V.) administration sets to deliver liquid medication to patients. When the patient needs medication, such as an antibiotic, standard practice until recently has been to deliver such a drug by a "piggy-back" drip into the primary infusion line. Recently, however, the procedure for delivering antibiotics and other drugs to patients on I.V. therapy has been changing. Mechanically driven syringes, frequently called or referred to as syringe pumps, are available to hospitals and other users for the administration of drugs and other liquid medications which the patient may require.
Indeed, a wide variety of syringe pumps and other liquid medication delivery devices are becoming available. With particular reference to a syringe pump, as that term is used herein, such pump employs a conventional or modified syringe which holds the liquid medication, the plunger of which is typically driven or pushed by a mechanized element for the automatic delivery of the liquid contents within the syringe. These syringe pumps are now available in battery powered form, include microprocessor technology for programming rates or times of medication delivery, have digital displays for ease of viewing by the user or the patient, include alarm circuits in the event of duty cycle completion, failure, or line occlusion, and other features to facilitate the operation or performance of the syringe pump. As these syringe pumps become more sophisticated, the technological advances frequently have the tendency to increase the expense of making and using the device, increase the complexity of use, and increase the opportunities for breakdown and repair if one or more of the operational elements fail.
For example, many existing syringe pumps include an occlusion circuit for producing an alarm (visual and/or audible) when the syringe cycle has been completed, if the I.V. line or syringe is occluded or if there is a system failure. One common mechanism for activating the occlusion circuit has been the use of one or more switches which are tripped after the syringe plunger passes a certain point during its travel into the syringe barrel. In other instances, the syringe is mounted on a spring loaded platform which is subject to a small linear movement after a certain force level has been reached. Thus, if the plunger being pushed into the syringe barrel either reaches the bottom of the syringe or cannot expel the liquid contents due to an occlusion in the line, the force against the plunger causes the entire spring-mounted syringe to move in linear fashion. This movement, in turn, either trips a switch or may be sensed by position sensitive sensors to send a signal to the occlusion circuit so that the automatic operation of the syringe pump may be terminated.
Improvements in such a plunger movement monitoring circuits are not only desirable, but are still being sought, in order to reduce the expense of the elements heretofore needed in such an occlusion circuit, as well as to eliminate the need for switches or sensing elements in order to achieve the desired results and make the syringe pump fail safe.
Rate feedback measurements for syringe pumps have been obtained from a tachometer gear driven off of the injector drive motor. The output voltage from the tachometer corresponds to the velocity of fluid flowing due to the pressure generated in the syringe pump. In the event that the tachometer became disengaged from the motor or the motor stopped, the voltage produced could approach zero and be indistinguishable from noise and interference, thus resulting in errors and inaccuracies.
In order to overcome the problems of velocity of flow measurement obtained from a gear driven tachometer, artisans have used a plunger position feedback signal generated by a potentiometer mechanically driven by the syringe motor. Such systems have been designed to use a rotary potentiometer which is drivingly connected to the motor output, such that as the motor shaft turns a corresponding gear drives the potentiometer to produce a direct measurement of shaft rotation. Since the motor is connected through means of a ball nut to the syringe plunger, the potentiometer should properly read the position of the plunger. This is not the case however if an overload conditions occurs or if for some reason the motor gear drive to the potentiometer fails. Therefore, it is desired that the gear drive interconnection of the potentiometer be eliminated in order to be certain that the exact position of the plunger is known.
It is toward such improvements, as mentioned already, as well as other improvements as will be pointed out, that the present invention is directed.